MOS capacitors are made in standard bipolar integrated circuits by using the emitter diffusion for the lower plate, an oxide grown over the emitter diffusion for the dielectric, and a top plate of aluminum also used for interconnecting the components of the circuit. The advantages of this capacitor are that it requires only one mask and one additional oxidation in addition to the steps required to manufacture a standard bipolar integrated circuit. The disadvantages of this capacitor are that its lower plate has high capacitance to the substrate, and it is sometimes electrically leaky to the substrate. These disadvantages were in part overcome by a capacitor using polycrystalline silicon for the bottom plate (U.S. Pat. No. 3,864,817, Lapham, Jr. et al.).
The standard capacitor cannot be made with circuits that have "washed" emitters because the lower plate structure consisting of emitter diffusion with overlying oxide dielectric is impossible to make. "Washed" emitters are those that have no separate contact masking; the emitter opening constitutes the contact, and it will only function as such with the absence of insulating oxide in the contact.
Further improvements, particularly for MOS applications, have evolved. For example the "double poly" capacitor has both plates made of polycrystalline silicon. This structure, while more complicated, offers low voltage linearity of capacitance and compatibility with MOS circuits. A variation on this structure is the metal oxide polycide capacitor of U.S. Pat. No. 5,086,370 (Yasaitis).